It is known, e.g. from French patents Nos. 1,195,279 and 1,307,410, to provide the light source with a slit diaphragm and with a projector which images that slit on a predetermined point in line with the optical axis of the microscope, namely a point located during examination in the zone of the patient's eye that is to be observed. By a pivotal movement of the projector, trained upon a mirror, the angle of incidence of the beam upon the zone of observation can be varied.
In these known instruments it is necessary to take into consideration, regarding the location of the pivotal axis, both the mechanical requirements and the optical conditions to be satisfied; compromise solutions adopted in the past tend to complicate the system. Moreover, with the pivotal axis and the projector disposed at opposite sides of the mirror, the necessary mounting elements unavoidably obstruct part of the field of view.